The technical field of the invention pertains to photodetectors and, in particular, to waveguide-based high-speed photodetectors.
There are many lightwave applications, such as optical telecommunications and chip interconnects, that involve transmitting optical signals and converting them to electrical signals at high data rates. Systems for performing such transmission and conversion usually require a photodetector compatible with the speed and bandwidth of the optical signal. Preferred photodetectors are typically PIN (p-type/intrinsic/n-type) semiconductor (e.g., Si or Ge) detectors, as such detectors can have a fast (i.e., GHz) frequency response.
Certain high-speed photodetectors utilize optical waveguides as a conduit for providing light to the intrinsic region of a PIN photodetector. An optical waveguide is a planar, rectangular or cylindrical structure having a high-index core surrounded by a low-index cladding. Light is trapped in the waveguide mostly within the high-index core, with a small portion of the light propagating in the cladding as an evanescent wave. When the intrinsic region of a PIN photodetector is located sufficiently close to the optical waveguide, light can be coupled to the intrinsic region via the evanescent wave. This phenomenon is referred to as xe2x80x9cevanescent coupling.xe2x80x9d
To form a high-speed waveguide-based photodetector, the light traveling in the optical waveguide must be efficiently coupled to the intrinsic region of the photodetector. This light is then converted to photon-generated carriers, which then diffuse out to the electrodes (i.e., the p+ and n+ regions of the PIN detector). The result is an electrical signal (e.g., a photocurrent) that corresponds to the detected light.
The speed of the detector is related to the time it takes for the photon-generated carriers to reach the electrodes. This time is referred to as the xe2x80x9ctransit time.xe2x80x9d The narrower the intrinsic region, the shorter the transit time and the faster the detector. A fast photodetector allows for the detection and processing of high-speed optical signals.
Often, the width and length of the intrinsic region of a photodetector is dictated by the width and length of the waveguide. However, the waveguide is typically designed for optimally transmitting a particular wavelength of light rather than for optimizing the detector speed. For low-index waveguides, the intrinsic region width can be quite wide (e.g., greater than 1 micron) and also quite long (e.g., greater than 50 microns).
The intrinsic region of a PIN detector is typically silicon (Si) or germanium (Ge), both of which have a high refractive index (e.g., about 3.5) as compared to the typical optical waveguide index (e.g., about 1.5 for SiOxNy). This results in a mismatch between the optical waveguide and the PIN detector with respect to the propagation constant and the waveguide mode of the guided lightwave. This mismatch leads to inefficient optical coupling. In some cases, a relatively lengthy waveguide-detector interface may be used to make up for the coupling inefficiency and to ensure that sufficient light is coupled to the detector. However, a lengthy interface is undesirable because it results in a large detector. Further, in many cases, the interface length needed to make up for the coupling inefficiency is too long for practical purposes.